1. Field of the Invention
This invention relates generally to a method for etching silicon nitride on a semiconductor wafer, and more particularly to a method for etching silicon nitride which permits critical dimension gain by preferential plasma etching of silicon nitride over silicon dioxide and photoresist with a mixture of CHF3 and O2.
2. Brief Description of the Prior Art
Semiconductor processing often involves patterning a relatively thick layer of silicon nitride coating a relatively thin layer of silicon dioxide supported upon a silicon wafer substrate. The nitride layer is covered with a photoresistant material, and this masking layer is patterned with apertures in accordance with the desired silicon nitride pattern. Such a process is described in U.S. Pat. No. 4,484,979 (Stocker), which discloses a two-step method for patterning silicon nitride in a multifaceted etching chamber without penetrating through the underlying silicon dioxide layer. In the first step, relatively fast etching is accomplished through reactive ion etching (RIE) using trifluoromethane (CHF3) and oxygen (O2) in ratios of 1:1 to 5.7:1. In a second etching step, silicon nitride is more selectively etched with respect to silicon dioxide and photoresist, using a mixture of oxygen to CHF3 which is at least 9:1. Stocker accomplishes high selectivity of nitride to oxide etching in this second etch step, and thereby minimizes widening of apertures in the nitride layer (i.e. minimizes loss of linewidth control).
Barber et al., U.S. Pat. No. 4,966,870, describe selective reactive ion etching of silicon nitride over borophosphosilicate glass and titanium disilicide using a CHF3/O2 mixture. Baldi et al., U.S. Pat. No. 4,897,365, teach a method for reducing bird beaks (oxide wedges) formed during a planox process using selective etching of silicon nitride over silicon oxide with reactive ion etching using a CHF3/CO2 mixture. Silicon nitride patterning with photoresist and reactive ion etching of silicon nitride is well known to those skilled in the art, and the disclosures of Stocker, Barber et al. and Baldi et al. describe the such processes in substantial detail.
It is the primary object of the present invention to provide an improved method for removal of silicon nitride from a semiconductor substrate.
Another object to the present invention is to provide an improved method for removal of silicon nitride in preference to photoresist and silicon oxide in the processing of a semiconductor substrate.
A further object to the present invention is to provide a silicon nitride etch process which results in critical dimension gain.
Briefly, the preferred embodiment of the present invention is a method for plasma etching of silicon nitride using a mixture of trifluoromethane and oxygen in a ratio of approximately 8 to 1 to selectively etch silicon nitride in preference to silicon dioxide and photoresist.